In the prior art used aboard an undersea cable laying and cable recovery ship, cable tension is measured during cable laying and cable recovery operations. Classically as the ship is propelled through the water, the cable slides over a friction plate of a dynamometer and deflects the cable path. A fraction of the tension is detected by the resulting movement of the friction plate and a strain gauge in a load cell. The signal representing that fraction of the tension is amplified into a signal representing the full magnitude of the tension.
The process of sliding of the cable over the friction plate of a dynamometer is a source of error in the tension measurement. The metal friction plate provides a chute through which the cable slides. That plate wears as the cable slides through and is gouged and nicked as chains and fittings are handled. After these defects are incurred, the dynamometer produces erroneous tension readings because of slight changes in the cable deflection angle resulting from the wear and tear. Additional error in the tension reading is caused by the force of friction between the cable and the friction plate, as the cable slides over it. This force is greatly dependent upon the coefficient of friction between the cable and the friction plate. Error signals, arising from all of the aforementioned sources of error, are superimposed on the desired tension signal detected by the load cell. Together, all of these signals are amplified and applied to a readout device.
The resulting tension reading includes inaccuracies which are acceptable for undersea cables containing coaxial copper transmission media. Such inaccuracies are acceptable because copper is a malleable material that will stretch readily without breaking if a desired maximum tension is exceeded for brief periods.
Currently in undersea cables, optical fibers are being substituted for the coaxial copper transmission media. The optical fibers are much more fragile than the copper. When the ship is propelled through the water laying or recovering cable, the maximum allowable tension in the optical fiber cable is critical because the optical fibers stretch very little without breaking. Acceptable error in tension readings on optical fiber cables is very low. Thus it is necessary to substantially reduce the sources of errors encountered when making tension measurements with a dynamometer.